In recent years, as the hydrometallurgical plant of the nickel laterite ore, High Pressure Acid Leach using sulfuric acid has been attracted the attention. This method does not include pyro-type processing steps such as drying and a roasting step, and is composed of consistent wet-type processes, therefore advantageous in view of energy and cost, as well as in capability of obtaining a nickel-cobalt mixed sulfide with an enhanced nickel content of up to about 50 to 60% by weight.
The High Pressure Acid Leach to obtain the above nickel-cobalt mixed sulfide includes, for example, as exemplified in FIG. 3, a pretreatment step (1) for pulverizing and classifying the nickel laterite ore to make slurry; a leaching step (2) for obtaining leach slurry, by adding sulfuric acid to the resultant slurry to be subjected to High Temperature High Pressure Acid Leach at 220 to 280° C. under stirring; a solid-liquid separation step (3) for obtaining a leach solution containing nickel and cobalt (a crude nickel sulfate aqueous solution) and a leach residue, by solid-liquid separation of the leach slurry; a neutralization step (4) for neutralizing the resultant crude nickel sulfate aqueous solution; a zinc removal step (5) for precipitation removing zinc as zinc sulfate, by adding hydrogen sulfide gas to the crude nickel sulfate aqueous solution neutralized; a sulfurization step (6) for obtaining the nickel-cobalt mixed sulfide and nickel barren liquor, by adding hydrogen sulfide gas to the resultant zinc removed final solution; and a detoxification step (7) to detoxify the leach residue generating in the solid-liquid separation step (3) and the nickel barren liquor generating in the sulfurization step (6) (for example, refer to Patent Literature 1, page 2).
The hydrometallurgical plant of nickel laterite ore is composed of a boiler for obtaining steam to control reaction temperature of each step, a hydrogen sulfide manufacturing equipment for producing hydrogen sulfide gas to be used mainly in the steps (5) and (6), and still more a water facility, a power facility and a piping facility such as liquid sending pipes for coupling sequentially each step and the like, other than facilities required in the above-mentioned steps.
In operation of such a hydrometallurgical plant, because major intermediate products are in a liquid or slurry state, it is general that when supply of steam is stopped caused by a serious trouble of the boiler, the whole plant is stopped fundamentally, and after elimination of the serious trouble, or after repair, the whole plant is started-up again. Therefore, in order to operate the whole plant continuously and in high rate of operation, it is necessary to operate the facilities of each step or other facilities in a state of no trouble always.
Accordingly, in a practical plant, operation is made under constant load as much as possible to avoid load variation. In addition, prevention of unexpected occurrence of serious troubles is strived for, by increasing frequency of a scheduled shut-down, or extending the shut-down period to inspect the facility or perform repair as needed.
In addition, provided that, caused by a trouble generated at somewhere of the process, generation of such a state that load should be temporarily decreased (such a case is called “ramp down”) at the relevant and subsequent steps is unavoidable, usually, an emergency storage tank is installed, so as not to decrease throughput in such a case.
In this way, by keeping load as it is or decreasing load of the steps before the step where troubles occurred, and storing process liquid becoming excess in the emergency storage tank, and after the trouble is eliminated, load of the steps is increased (such a case is called “ramp up”) so as to treat the process liquid stored in the emergency storage tank repeatedly along with usual load, attainment of processing target amount per quarter term or annual term becomes possible.
However, such an emergency evacuation-type response is effective usually when time required in restore is within 1 shift (8 hours), although depending on size of the emergency storage tank, and when it is over that time, process liquid will be stored in the emergency storage tank up to a limitation level, resulting in inevitable operation stop.
By the way, as one of the serious troubles easily occurring in the pretreatment step (1) in a real plant, such a problem has been pointed out that in pulverization and classification of the nickel laterite ore mined, using a drum washer composed of a drum and a trommel in a coupled state, the nickel laterite ore short passes in the trommel, and is carried out to outside the system, and thus predetermined operation result cannot be obtained.
On the other hand, there has been proposed a method for preventing the trouble from occurring, by forming a protrusion having nearly rectangular cross-section shape on a circumference and satisfying specific requirements, as row of protrusion arranged in an interval of about 4 times sieve mesh size of the trommel, on said same circumference which becomes a perpendicular cross-section to a rotating axis of the trommel, at the inner face of the trommel of the drum washer composed of the drum and the trammel in coupling (refer to Patent Literature 2, pages 1 and 2).
In addition, as one of the serious troubles easily occurring in the sulfurization step (6), such a problem has been pointed out that in the step (6), the nickel-cobalt mixed sulfide generated adheres at the inner face of a reactor and grows as a scale, which impairs function of an facility or results in destruction thereof.
On the other hand, there has been proposed a method for preventing the scale from growing, by circulating use of nickel sulfide as a seed crystal, in the sulfurization step (6), in an amount equivalent to 4 to 6 times amount of nickel in the zinc removed final solution from the zinc removal step (5) (refer to Patent Literature 3, pages 1 and 2).
However, in the hydrometallurgical plant of the nickel laterite ore, because various troubles occur frequently taking the above troubles as representative examples, caused by such a process specific reason that hard ore particles are used as slurry, or the nickel-cobalt mixed sulfide is formed which is easy to adhere at the inner face of the reactor, it is general that operation rate of a real plant is not so high.
In recent years, there have been strong needs to increase production amount of the nickel-cobalt mixed sulfide, by increasing the throughput of the nickel laterite ore. To satisfy this requirement, it has been performed to install a plurality of lines of the treatment facilities of the steps (1) to (7). However, even when a plurality of lines are installed, because the above trouble occurs at each line in similar frequency, for example, in the case of two lines, not only merit of the two-line setting is not attained, but also there may be the case where even processing capacity of full operation of one line cannot be satisfied.
Under such a situation, it has been required early establishment of the hydrometallurgical plant of the nickel laterite ore which is, even when the serious trouble will occur, capable of minimizing influence thereof, and restoring a normal operation state early, after eliminating the serious trouble, and an operation method thereof.